Biology Semester II |
 |
|
| Sections: |
Introduction | Section 1 | Section 2 | Section 3 |
| Section Three: |
Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 |
 |
Biology: Plant Hormones, Nutrition, and Transport: Part Seven Water and Mineral Uptake Animals have a circulatory system that transports fluids, chemicals, and nutrients within the animal body. Some plants have a similar system. Vascular plants have a vascular system and nonvascular bryophytes have trumpet hyphae. Vascular systems of plants are composed of xylem and phloem cells. In bryophytes, the soft tissue cells, the trumpet hyphae, transport materials within the plant body. However, these trumpet hyphae are not as efficient as vascular tissues. This inefficiency limits the height of bryophytes to only a few inches, as opposed to the hundreds of feet trees can reach. This discussion of water transport will focus on how water passes into the roots and up the stems of vascular plants. As you might expect, we begin at the roots!
The endodermis has a strip of waterproof material known as the Casparian strip. (This is the same material that allows cork to repel water.) This strip wraps around each endodermal cell like a ribbon. It forces water through the endodermal cell cytoplasm, allowing the endodermis cells to regulate the amount of water that reaches the xylem. As water is pulled upward through the root xylem cells, water from the phloem and endodermis will flow into the xylem. Only when water concentrations inside the endodermal cell fall below that of the cortex cells does water flow into the endodermis and into the xylem.
|
|
© 2009 Aventa Learning. All rights reserved. |
The vascular plant root has an epidermis, a cortex of parenchyma cells, and a vascular cylinder of phloem surrounding a solid core of xylem. Root hairs provide increased surface area and thus more efficient absorption of water and minerals. The parenchyma of the cortex often stores excess sugars as starch. A unique tissue layer, the endodermis, marks the boundary between the cortex and the vascular tissue. This layer acts as a gateway for water flowing into the vascular tissue cells. Phloem, as you may remember, is involved in transporting sugars, hormones, and water within the plant. The cells of the xylem tissue move water and dissolved minerals up through the plant.
Water and dissolved mineral nutrients enter the plant’s vascular tissue via two routes. Through the intracellular route, water and selected solutes pass through the cell membrane of the epidermis of the root hair. They then pass through plasmodesmata on every cell until they reach the xylem. In the extracellular route, water and solutes enter the cell wall of the root hair and pass between the wall and plasma membrane until they encounter the endodermis, a layer of cells that they must pass through to enter the xylem.
